If it is going to be implemented, (and other airports do it), it would need to be after the public transport improvements have been completed, especially from the south.
That's what the article says:
"Once improvements to public transport to the airport have been delivered we believe there may be a case for a congestion charge"
Also, the first ten minutes needs to be free.
That would rather defeat the object of the exercise. Again, from the article:
"Heathrow said a congestion charge would help to 'discourage drop off and pick up'"